Solvent-cleaning process



Filed NOV. 18, 1964 INVENTOR. CLETE M. 5M ITH BY @wfq gum.

United States Patent O 3,434,881 SOLVENT-CLEANING PROCESS Clete M. Smith, Pittsburgh, Pa., assignor to PPG Industries, Inc., a corporation of Pennsylvania Filed Nov. 18, 1964, Ser. No. 412,131 Int. Cl. B08b 9/08, 3/08 U.S. Cl. 134-22 3 Claims ABSTRACT F THE DISCLOSURE face.

The present invention relates to solvent cleaning. More particularly, the present invention relates to the improvement in the method of cleaning vessels with organic solvents in a rapid and eicient manner.

In cleaning vessels, such as tank cars, tank trucks, storage tanks, barges and the like, the use of vapor cleaning with solvents such as perchloroethylene, trichloroethylene, methylchloroform and other similar organic solvents has achieved considerable success. These solvents have been found especially effective in removing substances such as mineral oils, animal oils, fats, resins, plasticizers, asphalts, and other difficult to remove substances from the surfaces of vessels employed to store or transport these materials. Thus, it is known in the art to employ a vapor generator which serves to vaporize a solvent such as perchloroethylene for example and feed it to the storage tank which contains on its surface material which it is desired to remove prior to placing another material in the vessel or to clean the vessel prior to storing similar material in it once again. An example of such a system is shown in Chemical Wee magazine, May 7, 1960 issue, on page 26.

While the vapor generator method of cleaning internal surfaces of tanks provides a rapid and efficient method of cleaning the surfaces, it does possess certain drawbacks. which are unattractive. Thus, large headers are employed to convey vapors from the vapor generating system to the tank or vessel which is to be cleaned. Since the cleaning is accomplished by the application of an atmosphere Within the tank of solvent vapors under atmospheric conditions of pressure some dilicult to remove materials which tenaciously adhere to the surface of the tank may be left |behind when the vapors have completely condensed and are removed from the system. In some operations, depending upon the contour and size of the tank involved, considerable periods of time are usually required before the atmosphere of vapors cleans all the surfaces of the tank.

In accordance with the instant invention a rapid method is provided for eiciently cleaning the surface of a storage vessel or a vessel which is employed to transport material. The invention involves a flashing of solvent vapors within the storage vessel to generate an atmosphere of vapors. The invention also provides for a flashing at the surface of the tank and a direct hot solvent spray on 3,434,881 Patented Mar. 25, 1969 ICC the tank surface to quickly dissolve the material adhering to the surface of the tank. The system of the present invention also provides a solvent rinse during the treatment as well as a condensation rinse by an atmosphere of vapors within the tank itself. Operating in the fashion hereinafter described a rapid and complete cleaning of storage vessels, tank trucks, tank cars, barges, and the like may be achieved to a degree heretofore believed not to be possible.

In accordance with the present invention the liquid solvent to be employed for the purpose of cleaning a vessel is pumped under pressure through a heat exchange element. In its passage through the heat exchange element the temperature of the solvent is raised preferably 50 to higher than its boiling point at atmospheric pressure. The material is then passed from the heat exchanger through preferably insulated piping to a nozzle element or orifice which has a considerable back pressure applied thereto. The solvent is pumped at such a rate that it provides in the heated solvent pressures on the order of 50 to 300 pounds per square inch gauge. The exact pressure employed will be sufficient to overcome the back pressure of the orice used in the cleaning vessel itself, so that the pressure will actuate the nozzle or orice and permit the spraying directly into the tank of the superheated liquid solvent. Upon entering the vessel to be cleaned the hot liquid solvent flashes to a considerable extent as it is emitted from the orifice or nozzle employed, while some flashing of the solvent occurs at the surface of the tank. The flashing that occurs on the surface of the tank literally explodes the scum off the surface of the tank at the point of contact. The flashing of the solvent stream as it is transmitted from the orice or nozzle to the sides or Walls of the tank or vessel generates vapors and provides an atmosphere of solvent vapors within the tank itself. The hot liquid is sprayed at high temperature on the surface of the tank to promote quick, high temperature dissolving utilizing the hot solvent material on all of the surfaces of the tank touched `by the liquid spray. The liquid spray runs down the wall to the vessel thus providing a rinsing action as it falls to the bottom of the vessel. The condensing vapors provided by the atmosphere of vapors within the tank as they condense on the walls and bottom of the vessel also provide a further rinse.

In the operation of the system of the present invention it is preferable to employ as the orifice or nozzle an element which rotates in at least two planes so that the liquid stream of hot solvent entering the vessel is rotated on the `walls of the vessel in a horizontal as well as a partially vertical direction. This provides for the maximum utilization of hot liquid and facilitates the application of this hot liquid to all surfaces of the storage vessel or tank. While a movable nozzle or orice is preferably employed of course it is to be understood that any liquid dispersing device may be employed which will provide a maximum spraying effect upon the vessel to be cleaned. The important consideration is to feed to the dispersing device in the tank a hot liquid solvent under pressure so as to distribute it as rapidly as possible and to all parts of the vessel to be cleaned so that as much of the `wall surface of the vessel as practicable may 'be contacted with hot liquid solvent for the purpose of cleaning. l

The solvent material after the cleaning operation is removed from the tank and the scum or sludge removed therefrom. The solvent is then regenerated for further use in the cleaning process.

For a more complete understanding of the present invention reference is made to the accompanying drawing -which is a schematic illustration of the method of the instant invention as applied to the cleaning of a storage tank.

In the drawing the tank 1 represents a storage tank for clean trichloroethylene. The trichloroethylene is fed through line 2 through a pump 3 and line 4 to a heat exchange element 5. The pump 3 is designed to provide a supply of liquid at a pressure sufficient to activate the spray element 8 located in the vessel or storage tank 9 to be cleaned. The hot liquid under pressure is fed through the heat exchanger 5 which is operated to heat the liquid solvent in its passage there through to a temperature of between 240 to 300 F. The hot liquid solvent is then transported through lines 6 and 7 to the spray element 8. The hot liquid solvent omitted by the spray element 8, which is a rotating nozzle, is sprayed on the inner surfaces of the tank in such a manner that all portions of the tank are contacted with the hot liquid solvent during the spraying operation. The solvent containing the removed scum or sludge material adhering to the surfaces of the tank is removed through line 10 and taken to a dirty storage tank 11. In this tank sludge accumulating on the bottom of the tank is removed from line 12. The solvent is then transported through line 13 to a still 14 where the solvent is distilled and the vapors of the solvent are taken overhead, condensed in condenser 16 and then introduced into the trichloroethylene storage tank 1 as liquid. Sludge material accumulating in the still pot are removed through line periodically.

As can be readily seen the operation of this system is compact and the equipment requirements are simple. The

liquid solvent which is provided for the cleaning of the storage `vessel is hot, that is at least 50 higher than its normal boiling point, and it is introduced under pressure so that the advantages of hot lisuid solvent cleaning as well as flash vaporization on the surface of the vessel and the creation of an atmosphere of solvent within the storage vessel caused by instantaneous ashing of the solvent produced provides the maximum amount of solvent in a given period of time for cleaning the surfaces of the vessel.

As has been mentioned it is desirable in raising the temperature of the solvent to raise this temperature approximately 50 F. higher than its normal boiling point. Thus, for a trichloroethylene Awhere the boiling point of the trichloroethylene is approximately 189 F., Y

the temperature is raised at least 50 F. to 150 F. and may range broadly between 240 F. to 340 F. depending upon the equipment employed. Thus, if desired temperatures considerably in excess 0f 300 F. can be employed `with trichloroethylene. Similarly with perchlorof ethylene being utilized as a solvent where the 4boiling point is approximately 250 F. the temperature of the perchloroethylene leaving the heat exchanger should approximate at a temperature of 300 F. and may range broadly between 300 to 450 F. In a similar fashion with rnethylchloroform as a solvent which boils at approximately 165.2 F. an exit temperature from the heat exchanger of approximately 220 F. is attained in the solvent stream and this may range broadly from between 220 F. to 325 F. or more. In similar fashion, depending upon the organic solvent employed, the solvent entering the heat exchanger has its temperature raised at least '50 F. in the heat exchanger and prior to its admission to the storage tank or vessel to be cleaned.

In similar fashion the pressure at which the liquid is sprayed into the vessel to be cleaned is subject to considerable variation and will depend upon the type of material being removed, its adherence to the tank walls, and other similar considerations. The pressure is normally controlled by the pump utilized to feed materials to the heat exchanger and the back pressure on the nozzle or spraying device utilized in the vessel. Pressures ranging between 50 p.s.i.g. to 350 p.s.i.g. are contemplated but higher pressures may be employed if desired. Preferably the pressure is adjusted to provide pressure of about p.s.i.g. for the liquid spray entering the vessel through the nozzle or spray device employed. The particular type of spraying device `used is of no particular significance so long as the device employed provides a spray ywhich will encounter all surfaces of the vessel to be cleaned during the feeding operation of the solvent to the vessel. Thus, shower sprays, spinners, rotating nozzles and other llike devices may be employed. It is preferable to utilize a nozzle that sprays material into the tank in the form of a stream having more than one rotating axis so that maximum coverage of the surface to be cleaned is attained in a short period of time.

The particular type of heat exchanger employed is of no consequence provided it is adequate to impart to the liquid fed to it sufficient heat at the rate at which the liquid is fed to raise the temperature of that liquid at least 50 F. higher than its normal boiling point. Thus, shell and tube type heat exchangers, helical coil exchangers and other similar devices well known in the art may be employed.

While the invention has been described with reference to the cleaning of a storage tank utilizing trichloroethylene as a solvent, it of course is to be understood that the other solvents such as methylchloroform, perchloroethylene and the like may be employed as a solvent. In all applications of this instant invention to the solvent in `vessels the important considerations are that the solvent temperatures be raised sufficiently to impart to it temperatures in excess of the normal boiling point of the solvent. The solvent further is fed to the storage vessel as a liquid stream of solvent under pressure so that it strikes the surface of the tank at sufficient pressure to mechanically assist in the cleaning of that contacted surface.

While the invention has been described with reference to certain specic embodiments it is to be understood that it is not to be limited in any way except insofar as appears in the accompanying claims.

I claim:

1. A method of cleaning an inner surface of a storage vessel with organic solvent comprising heating liquid organic solvent under superatmospheric pressure of at least 50 p.s.i.g. to a temperature of at least 50 F. higher than its boiling point at atmospheric pressure to superheat the liquid solvent, introducing the superheated organic solvent into the vessel, flashing some of the hot liquid solvent entering the vessel, and also impinging some of the hot liquid solvent directly against the surface, ashing said impinged solvent at said surface and removing said solvent and material cleansed from the surface from said vessel.

2. The method of claim 1, wherein the organic solvent is a member of the group consisting of perchloroethylene, trichloroethylene and methylchloroform, the solvent is heated to a temperature of from 50 F. to 150 F. higher than the boiling point of the solvent at atmospheric pressure, and the heated solvent is introduced at a pressure of from 50 and 300 p.s.i.g.

3. A method of cleaning a surface in a storage vessel with organic solvent comprising heating liquid organic solvent under superatmospheric pressure of between 50 and 300 p.s.i.g. to a temperature between 50 F. and 150 F. higher than the boiling point of the solvent at atmospheric pressure whereby to superheat the liquid solvent, introducing the superheated organic solvent into the vessel, flashing some of the hot liquid solvent entering the vessel, imparting hot liquid solvent directly against the surface in the vessel and ashing said impinging solvent at said surface to remove material adhering to the surface and removing the solvent along with material cleansed from the surface from the vessel.

References Cited UNITED STATES PATENTS Dougherty 134.42 X

3,046,163 7/1962 Kearney 134-24 X 3,079,286 2/1963 Kearney 134-12 FOREIGN PATENTS 548,116 9/ 1942 Great Britain.

FRANK W. LUTTER, Primary Examiner.

US. C1. X.R. 134-12 

